The invention proceeds from a flat discharge lamp in accordance with the preamble of Claim 1. The invention also relates to a method in accordance with the preamble of the method claim for producing this discharge lamp.
The term xe2x80x9cdischarge lampxe2x80x9d in this case embraces sources of electromagnetic radiation based on gas discharges. The spectrum of the radiation can in this case comprise both the visible region and the UV (ultraviolet)/VUV (vacuum ultraviolet) region as well as the IR (infrared) region. A fluorescent layer can also be provided for the purpose of converting invisible radiation into visible radiation.
It is also a case here, in particular, of flat discharge lamps with so-called dielectrically impeded electrodes. Here, the dielectrically impeded electrodes are typically implemented in the form of thin metal strips which are arranged on the outer wall and/or inner wall of the discharge vessel. If all electrodes are arranged on the inner wall, at least some of the electrodes must be completely covered off from the interior of the discharge vessel with the aid of a dielectric layer. Discharge lamps of this type are disclosed, for example, in EP 0 363 832 (FIG. 3) and German Patent Application P 197 11 892.5 (FIGS. 3a, 3b). Flat discharge lampsxe2x80x94also denoted as flat radiatorsxe2x80x94have a discharge vessel which is formed from a base plate and cover plate, for example made from glass, which are interconnected via a frame.
It is possible to dispense with a frame when the base and/or cover part is not flat but formed such that a discharge vessel is already formed upon assembling only the base part and cover part. For example, the base part and/or the cover part can be shaped like a trough, for example by deep drawing. In this case, as well, for very large-area flat lamps the predominant fraction of the shaped base part and/or cover part is at least approximately flat and therefore requires one or more support points for stabilization.
Consequently, the terms base plate and cover plate are also to be understood below as structures which are actually, if appropriate, not completely but at least predominantly (approximately) flat.
The discharge vessel contains a gas filling of defined composition and filling pressure, and must therefore be evacuated before filling. Consequently, the discharge vessel must permanently withstand both underpressure specifically during the production of a lampxe2x80x94and the later filling pressure, which is less than atmospheric pressure, for example between 10 kPa and 20 kPa. According to specification from glass manufacturers, this time-load withstand strength is to be set with a value of approximately 8 MPa and is yielded, as a function of the glass thickness used, from the maximum sag over a length between two supports. The latter is inversely proportional to the glass thickness used and at a specific temperature is, moreover, a function of the pressure difference between the interior of the discharge vessel and the outside.
Consequently, for a given pressure difference and temperature the time-load withstand strength can be achieved even in the case of thin glasses by shortening the length between two bearings. Use is made for this purpose of support points which are arranged over the base area of the discharge vessel adequately in terms of position and number. The mutual spacing between immediately adjacent support points is dimensioned such that the targeted length is not exceeded at any point.
The support points usually comprise a glass rod section, a glass ring, a glass tube or a glass ball whose heights, or respectively diameters correspond to the frame height. To date, the support points have been bonded to the base plate and/or cover plate by means of a suitable sintered glass. In this case, the bonding fixes only the support points, but is too thin to be able to compensate the differences in height.
Furthermore, recessed glass webs are also known. One possibility consists in processing a solid material by sandblasting such that the base plate is produced with an appropriate thickness and the glass webs are produced with the desired shape and height. In this case, as well, the cover glass and glass web are joined by bonding by means of a sintered glass.
A further possibility consists in integrating the webs by heat treatment of the glass such as, for example, deep-drawing by means of underpressure or dead weight or pressing. Here, the base plate or cover plate is heated above its softening point and shaped by means of a mould using standard methods.
It is the object of the present invention to provide a flat discharge lamp in accordance with the preamble of Claim 1 with improved support points.
This object is achieved by means of the features of Claim 1. Particularly advantageous refinements are to be found in the claims dependent thereon.
A further object consists in specifying an improved method for producing flat discharge lamps.
This object is achieved by means of the features of the method claim. Particularly advantageous refinements are to be found in the claims dependent thereon.
A precondition in the case of the considerations set forth at the beginning for stabilizing the discharge vessel by means of support points is that adequate, double-sided contact is ensured with the base plate and cover plate for all support points. Specifically, if a support point does not have two-sided contact, it fails as an effective support. Consequently, the free length in the region of this ineffective support point is doubled, viewed radially starting from the latter and in the plane of the plates. The targeted length is therefore exceeded in some circumstances and the time-load withstand strength is impermissibly reduced. It is necessary for this reason, also to take account of and even out irregularities in the base plate and cover plate over the basic area of the discharge space, as well as frame height tolerances. This gives rise to increased difficulties, particularly in the case of a rising number of support points and an increasing area.
The invention proposes support points which each comprise two components. These two components are distinguished in that they have clearly differing viscosities during joining of the vessel, that is to say at the jointing temperature. Here, one component works as a so-called xe2x80x9chard partxe2x80x9d and has a very high viscosity, typically more than 109 dPa s, preferably 1011 dPa s and higher at the jointing temperature. A soda-lime-silica glass is suitable, for example. The other component works as a so-called xe2x80x9csoft partxe2x80x9d with a low viscosity, tuned to the jointing temperature, in the region of typically 103 dPa s to approximately 105 dPa s or less, that is to say a defined and nondestructive deformation is provided precisely at the jointing temperature, possibly with a slight application of force. Sintered glass or lead glass, for example, is suitable.
In principle, virtually any shapes are initially conceivable for the xe2x80x9chardxe2x80x9d part, in particular including bar sections, tube sections, tubes, balls, rings, bars and the like. In principle, virtually any shapes are likewise suitable for the xe2x80x9csoftxe2x80x9d part, for example rings, pieces of a cylinder, truncated cones, platelets or troughs. However, in each case the shapes of the two components are to be suitably coordinated with one another. The viscosities of such sintered glass parts are produced by tuned mixing ratios of different sintered glasses, using standard methods such as, for example, pressing, casting, extruding and subsequent sintering. The possibility exists here of producing these sintered parts immediately with the desired dimensions, or of fabricating them by machining ahead of the sintering. Moreover, the two components can also respectively be connected from the start to form a single-piece support point.
The mode of operation of a two-component support point during production of a flat discharge lamp consists in that its height before joining of the lamp deliberately exceeds the height of the frame, for example, in the range from a tenth of a millimetre to a few millimetres, in particular between approximately 0.5 mm and 2 mm. The cover part therefore bears only against the support points before the joining operation. When the jointing temperature is reached, the xe2x80x9csoftxe2x80x9d part of the support points in each case attains a sufficiently low viscosity, typically of the order of magnitude of 107 dPa s, to be deformed under the weight of the cover plate. The cover plate sinks in this case onto the frame, coated with solder, for example glass solder, and fuses together with the latter. The lamp is sealed in this way.
The great advantage of this mode of procedure is as follows. Since the cover plate always bears against all support points in this operation, each support point is levelled to the required height, that is to say exactly such that each support point makes double-sided contact with the base plate and cover plate.
In addition, this property of the two-component support point permits the lamp to be produced without an exhaust tube in a vacuum furnace filled with filling gas at filling pressure, since the lamp closes automatically as the jointing temperature is reached.
In the extreme case, the support points can also respectively comprise to a decidedly predominant extent a component of low viscosity and a separating layer of high viscosity as second component which prevents the component of low viscosity from sintering onto the base plate or cover plate. Fluorescent material can also serve as separating means.